Sensor Development: Metrology Tools for Climate Science

Sensor Development: Metrology Tools for Climate Science

250 spectra in 0.7 s

David A. Long

Adam J. Fleisher, Zachary D. Reed, and Joseph T. Hodges

Greenhouse Gas and Climate Science Measurements Seminar

1 August 2014

Our approach

• Both in situ monitoring and remote sensing require cutting-edge spectroscopic measurements

• We develop novel techniques which allow for enhanced:

• Spectral coverage

• Portability

• Sensitivity

• Accuracy

• Speed

• Selectivity

Outline

• A series of new spectroscopic techniques

• They offer a range of complexity, speed, and sensitivity

• Specifically I will discuss:

• Photoacoustic sensor

• Ultrasensitive cavity ring-down instruments

• Multiplexed detection with optical frequency combs

NIST photoacoustic sensor

• Designed for routine monitoring of CO2 concentrations

• Will be placed on the top of the Admin. Building at NIST (101).

Figure from HDR Architecture, Inc.

Photoacoustic spectroscopy (PAS)

• Zero background

• Optically broadband

• Relatively easy to implement

• High sensitivity • signal scales with laser power

• Wide dynamic range

Photoacoustic spectrometer at 1.6 µm

• Allows for routine, automated measurements of ambient CO2

• Uncertainty of only 0.8 ppm (0.2%)

Z. D. Reed et al., Appl. Phys. B, (2014).K. A. Gillis, D. K. Havey, J. T. Hodges, US Patent Pending.

New 2 µm sensor

• Moved to 2 µm to probe stronger CO2 transitions– Able to utilize recently developed laser technology

• Signal-to-noise ratios of ~14,000:1 for CO2 at ambient levels

• Allows for simultaneous humidity measurements with the same laser

Future directions for the PAS instruments

• Dual species PAS instrument– Use fiber switches to probe numerous species simultaneously– Potential targets include CO2, H2O, NH3, and CH4(and their isotopologues)

• Mid-infrared PAS– Would allow us to probe CH4 and C2H6 simultaneously– Allows for source attribution of CH4 emissions

Outline







Single-mode excitation with locked cavity

Cavity ring-down spectroscopy (CRDS)

incident laser beam

recirculating field

detector

low-loss dielectric mirror

ring-down signal

Advantages:

•High effective pathlength and sensitivity •Insensitive to laser intensity noise•Small sample volume

Empty-cavity

Absorbing medium

t

The problem

frequency

• To record a spectrum you need to tune the laser frequency

• This generally requires thermal or mechanical tuning

• This is usually non-linear and very slow

• Things are even more difficult with cavity-enhanced spectroscopy (discrete frequencies)

Frequency-agile, rapid scanning (FARS) spectroscopy

Advantages:• Overcomes slow mechanical and thermal scanning • Links optical detuning axis link to radio-frequency (RF) standards• Wide frequency tuning range (> 130 GHz = 4.3 cm-1)

Method:• Use waveguide electro-optic phase-modulator (EOM) to generate tunable sidebands• Drive PM with a rapidly-switchable microwave (MW) source• Fix carrier and use ring-down cavity to filter out all but one selected side band

G.-W. Truong et al., Nature Photonics, (2013).D. F. Plusquellic, et al., US Patent US20130228688 A1 (2013).

MW source

EOM

cw laserring-down cavity

side-band spectrum

Detector

gas analyte

wC+d+2wf

wC+d+wf

wC+d

cavityresonances

frequencyscanning

wf

FARS operating principle

G.-W. Truong et al., Nature Photonics, (2013).

Very high acquisition rates: scanning

2 THz wide spectra recorded in 45 minutes

425 ppm of CO2 in air

ECDL grating moved every 12 GHz

Each point is the average of 100 RDs

Spectra of entire absorption bands

Lower finesse = faster rates

D. A. Long et al., Appl. Phys. B, (2013)

t = 30.85 ns 1.2 ns Finesse = 60RD acq. rate = 5 MHzst/t = 4 %

NEA = 1.9×10-8 cm-1 Hz-1/2

Finesse = 20,000RD acq. rate = 8 kHzst/t = 0.008 %

NEA = 1.7×10-12 cm-1 Hz-1/2

K. O. Douglass, JOSA B, (2013)

What if I want to scan faster?

• Entirely limited by the slow grating tuning

• So use higher bandwidth EOMs to reduce the number of grating steps

Even faster rates

Use recently developed W-band modulators (bandwidth up to 300 GHz!)

Image from Phase Sensitive Innovations

Even faster rates: single grating position

Allows for a 130 GHz (4.3 cm-1) to be recorded in 3 s.

Even faster rates: grating tuning

What if I want higher sensitivity?

• To reduce 1/f noise, we want to make the measurement away from DC

• Also need to rapidly compare ring-down time constants at different wavelengths

Improving the sensitivity: Heterodyne measurements

J. Ye and J. Hall., Phys. Rev. A, (2000)

• To do this we adapted the approach of Ye and Hall

Heterodyne measurements:Our approach

D. A. Long, et al., Appl. Phys. B, Under Review (2014)

• Replace their two AOMs with a single EOM

• This reduces the complexity and enables rapid scanning

t (µs)

Heterodyne measurements:Reaching the quantum-noise limit

• Utilized both a traditional InGaAs detector and an APD• Able to reach the quantum-noise limit with the APD• The traditional InGaAs allows for an NEA of 6E-14 cm-1 Hz-1/2

APD PIN


Most sensitive spectrometers

Technique Ref. Sensitivity (cm-1 Hz-1/2) Laser Tuning range (nm)

NICE-OHMS Ye et al. 1E-14 cw-Nd:YAG 0.1

HD-CRDS Long et al. 6E-14 ECDL 60

HD-CRDS Ye and Hall 3E-13 cw-Yb:YAG ~0.5

CRDS Spence et al. 1E-12 cw-Nd:YAG 0.14

FARS-CRDS Long et al. 2E-12 ECDL 60

NICE-OHMS Ehlers et al. 6E-12 Fiber laser 1

D. A. Long et al., Appl. Phys. B, (2013)


Heterodyne measurements:Rapid scanning

• Able to record a 26 GHz-wide spectrum in 17 ms with 50 GS/s AWG

• Observed a weak CO2 hot band transition (3.5E-25 cm molec.-1) in an air sample


Outline







What if I don’t want to scan?

• Then multiplex!

Multiplexed measurements: OFCs

OFCs have been used with a variety of detection schemes for spectroscopic measurements

Multiheterodyne

Dispersive (VIPA) Fourier TransformMandon, et al., Nat. Photonics (2009)Diddams, et al., Nature (2007)

Coddington, et al., Phys. Rev. Lett. (2008)

Mode-locked femtosecond OFCs

Advantages:• Wide bandwidth (octave-spanning)• Can be self-referenced (absolute freq. axis)

Disadvantages:• Essentially fixed repetition rate• Low power per tooth (nW to µW)• Large and expensive

Image from Menlo Systems

An alternate approach: electro-optic modulators

Ideal for targeted measurements of selected species

T. Sakamoto, et al., Electron. Lett., (2007).D. A. Long, et al. Opt. Lett. (2014).

Dual-drive MZM allows forpower-leveling of the comb

Variable pitch

Pitch can be changed in <100 µs

D. A. Long, et al. Opt. Lett. (2014).

Multiheterodyne spectroscopy

Common mode (no need for phase locking)

D. A. Long, et al. Opt. Lett. (2014).D. F. Plusquellic, et al. US Patent Pending (2014).

Referencing


Multiheterodyne spectra

Acquired in 30 s(average of 10,000 spectra)


Where are we headed?

• The mid-infrared

• Probing the strongest molecular transitions allows for the lowest detection limits

12CO2

14CO2

14CO2

Zoom in60,000,000,000X

Cavity ring-down spectroscopy in the mid-IR

• Instrument is up and running!

Image from Alpes Lasers

Detection limit of 3 ppt for N2O

Conclusions

• Presented several new techniques for rapid, ultrasensitive detection of GHGs

– Photoacoustic spectroscopy (PAS)• Relatively simply instrument allows for routine sensing

– Frequency-agile, rapid scanning (FARS) spectroscopy • Use an EOM to step scan the laser frequency• Scanning rates limited only by the cavity response time

– Heterodyne-detected cavity ring-down spectroscopy (HD-CRDS)• Make the measurement well above DC• Leads to quantum-noise-limited sensitivity

– Multiheterodyne spectroscopy with EOM-generated optical frequency combs• Allows for multiplexed detection of several trace gases• Far lower costs and complexity than with femtosecond lasers• Inherently common-mode

Acknowledgements

• Joseph Hodges, David Plusquellic, Adam Fleisher, Zachary Reed, Gar-Wing Truong, Szymon Wojtewicz, Katarzyna Bielska, Hong Lin, Qingnan Liu, Kevin Douglass, Stephen Maxwell, Roger van Zee– NIST

• NIST Greenhouse Gas Measurements and Climate Research Program

• NIST Innovations in Measurement Science (IMS) award

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Sensor Development: Metrology Tools for Climate Science